Each group of the type defined above is normally wrapped by feeding it into a respective U-shaped folding pocket travelling along a given wrapping path and bounded by two lateral walls and an end wall. The group is fed into the relative folding pocket together with a sheet of wrapping material, which, on inserting the group inside the folding pocket, is folded into a U so that two opposite parallel lateral portions contact the lateral walls of the folding pocket, and respective end portions project from the folding pocket. The group is then clamped contacting the end wall of the folding pocket by two end clamping members before said end portions are folded about the group and the end clamping members to form a tubular wrapping.
The known wrapping method described above produces substantially perfect tubular wrappings, i.e. rectangular parallelepiped-shaped to assist follow-up wrapping operations, when the cigarettes, e.g. 20 in number, are arranged in a 7-6-7 configuration, i.e. in which the two outer layers are the same width and either wider than or the same width as the middle layer.
The same does not apply, however, when the layer facing outwards of the relative folding pocket is narrower than the adjacent layer, i.e. when the 20 cigarettes are arranged, for example, in a 6-7-7 configuration, or when, for tax or cost reasons, for example, the group contains fewer than 20 cigarettes, and the layer facing outwards of the relative folding pocket comprises fewer cigarettes than the adjacent layer. In which case, the narrower outer layer defines, with the adjacent layer, at least one longitudinal lateral gap (normally two opposite longitudinal gaps) where the end portion of the relative lateral portion of the sheet of wrapping material is not supported when folded onto the outer layer of the group, so that the resulting tubular wrapping is not of the desired rectangular parallelepiped shape.